Two-stroke scooters emit more than 120 times the volatile organic compounds than other vehicles, say a European research team, who have dubbed them "super-polluters" and suggest that they should be regulated or even banned.

The scientists, who are affiliated to organisations in Switzerland, Italy, Germany, France and the UK, used various techniques, including mass spectrometry, to measure the emissions from the scooters, as they described in Nature Communications. They found that high concentrations of toxic aromatic compounds are present in the exhaust, making up about 40% of the total volatile organic compounds.

They include benzene at 300,000 µg/m3, which is way above the EU annual limit for the protection of human health at 5 µg/m3. So, motorists or pedestrians waiting behind an idling scooter will be in direct line for a massive burst of unhealthy exhaust gas.

It is these organic compounds which go on to form secondary aerosols which create the smog that we see in large cities and idling scooters produced the greater amounts of these aerosols.

Although scooters represent a relatively small proportion of road vehicles, the researchers estimated that they could contribute as much as 90% of all primary aerosols in Asian cities like Bangkok where there are a vast number of scooters on the road.

Banning scooters in cities would seem an obvious step and this is supported by data coming out of China. The city of Guangzhou, where scooters are banned, has far lower levels of roadside aromatic compounds than Donnguan, where they are not banned, even though the traffic volume is greater in the former city.